1. Field of the Invention
The present invention relates to bottom-layer, thermo-setting polymeric anti-reflective coatings used in multilayer photoresist systems, particularly to those having improvements in plasma etch rate, optical density, solubility in preferred coating solvents, feature coverage, and storage stability.
2. Background of the Prior Art
The effectiveness of bottom-layer polymeric anti-reflective coatings for achieving good critical dimension (CD) control in microlithographic processes is well established (U.S. Pat. No. 4,910,122). As device feature sizes have now decreased to the sub-0.5 micron regime, it has become imperative to use such anti-reflective coatings to inhibit standing wave formation within photoresist layers, eliminate backscattered light from microelectronic substrates, and reduce swing curve amplitude.
Current bottom-layer polymeric anti-reflective coating technologies have severe limitations for producing sub-0.5 micron feature sizes. Most notably, the intermixing between thermoplastic anti-reflective coatings (U.S. Pat. Nos. 5,234,990, 5,294,680, and 5,532,332, incorporated herein by reference) and overlying photoresist layers gives rise to discernible distortions at the base of the patterned features when viewed in cross-section. The magnitude of these distortions has now become significant because of the extremely small feature size, making any such distortion in the line shape unacceptable in device fabrication.
Thermosetting anti-reflective coatings exhibiting higher resistance to intermixing with photoresist layers have been introduced recently to overcome the limitations of thermoplastic anti-reflective coatings. Our co-pending U.S. patent application Ser. No. 08/517,089, entitled Thermosetting Anti-Reflective Coatings and Method, herein incorporated by reference, describes thermosetting anti-reflective coatings comprised principally of an oligomeric, hydroxy-functional resin; an aminoplast cross-linking agent; a protonic acid catalyst; and an appropriate solvent vehicle; wherein the hydroxy-functional resin is the reaction product of a phenolic or carboxylic acid dye with a low molecular weight epoxy resin having an epoxy functionality of 3 to 10. The coatings are cured by baking for 30 to 120 seconds at temperatures above 150.degree. C.
The curing mechanism of the thermosetting anti-reflective coatings disclosed in co-pending U.S. patent application Ser. No. 08/517,089 has a crosslinking mechanism similar to that of aminoplast-containing industrial coatings and photoresists such as those described in U.S. Pat. Nos. 3,744,904, 4,149,888, 4,316,940, 4,341,859, 4,478,932, 4,518,676, 4,734,444, 5,034,304, and 5,376,504, and herein incorporated by reference. It should be noted, however, that none of these prior art industrial coating nor photoresist compositions fulfills the requirements for anti-reflective coating applications, particularly with regard to providing sufficient light attenuation at typical anti-reflective coating thicknesses of 0.05-0.20 microns.
Although the dye-attached thermosetting anti-reflective coatings derived from low molecular weight epoxy resins provide many unexpected benefits, there are drawbacks. One such draw-back is their tendency to dewet on silicon dioxide, silicon nitride, and ion implanted microelectronic substrates, though this problem can often be corrected by appropriate formulation changes. A more serious drawback occurs upon plasma etching images into the anti-reflective coating layer. For example, for one preferred deep ultraviolet embodiment of our co-pending application, oxygen plasma etching proceeds at rates no faster than 1.25 times a thermoplastic polyarylethersulfone anti-reflective coating described in U.S. Pat. No. 5,234,990. Since polyarylethersulfone anti-reflective coatings are known generally to etch more slowly than deep ultraviolet photoresists, this implies that the thermosetting anti-reflective coating described in the co-pending application will etch at approximately the same rate as the photoresist during the pattern transfer step. Since the anti-reflective coating layer thickness is typically 0.05-0.10 microns, a significant negative etch bias may be observed at resist feature sizes below 0.30 microns unless the plasma etch process is highly anisotropic.
Fast-etching, thermosetting anti-reflective coatings derived from high molecular weight acrylic polymers have been disclosed. For example, European Patent Application No. 92118070.9 describes anti-reflective coating compositions which contain a dye-attached acrylic copolymer and an aminoplast crosslinking agent. However, the optical density, or light absorbing power, of such systems is limited by polymer insolubility at high levels of dye attachment. This is a critical limitation for mid-ultraviolet anti-reflective coatings since the known dye structures for this wavelength range have modest absorbance and must be present in high concentration to achieve an effective film optical density.
European Patent Application No. 93305124.3 likewise discloses thermosetting anti-reflective coatings which comprise at least one compound (typically, a polymer or an oligomer) having one or more glycidyl functions, at least one phenolic anthracene dye, and a solvent capable of dissolving these compounds. However, unlike the aminoplast-containing anti- reflective coating compositions discussed above, the title compositions must be heated for several minutes at high temperatures to insolubilize the coating. This long cure cycle reduces wafer throughput and makes the process generally unacceptable to manufacturers. In addition, the preparation of the title anti-reflective coatings, particularly that of the phenolic anthracene dye components, involves many steps, making the coatings too expensive to produce and use on a practical basis.
U.S. Pat. No. 5,597,868 discloses similar thermosetting anti-reflective coatings for 193 nm photolithography. A polyphenolic dye such as a novolac resin is combined with an acrylic polymer which has pendant epoxide functionality. Heating the coating results in a thermosetting reaction between the phenolic hydroxyl groups of the dye and the epoxide groups of the polymer. As claimed therein, however, the curing process must proceed for more than 10 minutes at temperatures greater than 170.degree. C. to be effective.